1. Field of the Invention
The present invention relates generally to a microwave oscillator and particularly to an improved microwave oscillator capable of stable oscillation even when a FET is used therein which is other than a package FET.
2. Description of the Prior Art
A conventional microwave oscillator has been disclosed, for instance, in Japanese Published Unexamined Patent Application No. Sho 57-26902.
FIG. 1 is a circuit diagram of the conventional microwave oscillator of the above-mentioned application, wherein an FET 1a is connected by its gate 11a to one end of a strip line 4a, the other end thereof being terminated by a dummy resistor 6a. The source 13a of FET 1a is connected to an output terminal 10a and also through a low pass filter 7a and a self bias resistor 8a to the ground. Drain 12a is connected to a power source terminal 3a and also to one end of openended transmission line 19a having a length of a quarter wavelength .lambda..sub.g /4. A dielectric resonator 5a is coupled to the strip line 4a.
In the conventional microwave oscillator of the above-mentioned configuration, the 1/4 wavelength open ended transmission line 19a grounds the drain 12a in microwave frequency. When a DC power source is supplied from the power terminal 3a, the potential of the gate 11a becomes lower than the potential of the source 13a, as a result of voltage drop caused by a current flowing through the self bias resistor 8a. According to the above-mentioned circuit configuration, oscillation is produced by cooperation of a negative resistance produced at the gate 11a and a resonance circuit consisting of the strip line 4a, and the dielectric resonator 5a coupled to the strip line 4a. The output of the oscillation signal is issued through the output terminal 10a.
In the above-mentioned prior art circuit configuration, however, when the FET 1a is configurated in a shape of a chip, or when the whole circuit is configurated in a monolithic integrated circuit, there arises a problem such that the negative resistance to be presented at the gate 11a is decreased, due to a decrease or elimination of the stray capacity which has existed in case of the prior art package 1. Therefore, the stability of the oscillation is lowered.